1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method that image a subject based on a magnetic resonance signal generated in the subject.
2. Description of the Related Art
In a magnetic resonance imaging apparatus that utilizes an iron shim to perform shimming, a static magnetic field intensity in an imaging region varies with a change in temperature of the iron shim. Such a variation in the static magnetic field intensity is called a B0 shift. Since a resonant frequency also varies when the B0shift occurs, a displacement of a reconstructed image may possibly occur. Such a displacement prominently occurs in an echo planar imaging (EPI) sequence (an echo planar imaging sequence) having a high frequency resolution in a phase encoding (PE) direction.
In EPI type diffusion weighted imaging (DWI), since a load of a gradient coil is large because of application of an motion probing gradient (MPG) pulse, heat generation from the gradient magnetic field coil is large. Therefore, an increase in temperature of the iron shim during scanning is large, and the B0 shift is also increased.
In order to avoid such an increase in the B0 shift involved by heat generation from the gradient coil, a measure for preventing an increase in temperature of the iron shim has been conventionally taken. Since this measure is realized by arranging the iron shim to be apart from the gradient coil or intensively cooling the iron shim, there is a problem in cost.
Thus, for example, JP-A 2005-270285 (KOKAI) proposes a technology of reducing an influence of the B0 shift without greatly modifying hardware, e.g., the iron shim.
However, the technology disclosed in JP-A 2005-270285 (KOKAI) is premised on a case where dynamic imaging using a field echo type EPI sequence (an FE-EPI sequence) is performed. Therefore, the technology disclosed in JP-A 2005-270285 (KOKAI) cannot be applied to a different sequence such as a spin echo type EPI sequence (an SE-EPI sequence).